RESPIRATORY GAS INTERCHANGE. 



353 



contains, if set free. The volume CO 2 given off is, however, only 

 about 4.38 to 4.5 volumes of O taken in, so that part of the O 

 must be used in some other way than in the manufacture of CO 2 . 

 7. The expired air is also said to contain traces of the following 

 impurities : (J) ammonia, (2) hydrogen, (3) carburetted hydrogen 

 (CH 4 ), (4) organic matter. These, and probably other impurities, 

 give the breath its peculiar odor and noxious properties, for an 

 atmosphere rendered "stuffy" by expired air is much more inju- 

 rious to health than an atmosphere in which a similar deficiency 

 of O or excess of CO 2 has been artificially produced by chemical 

 means ; this fact ought to be remembered when calculating the 

 ventilation required for hygienic purposes. The following table 

 may assist in comparing the atmosphere with the expired air : 



About ^ of the O which is used does not take part in the 

 production of the CO 2 , but this proportion may vary greatly. 

 Thus the estimation of the CO 2 can give no sure guide to the 

 amount of O taken up ; and each gas has to be estimated sepa- 

 rately if an accurate measurement be required. 



The average amount per diem may be said to be : 



Carbon dioxide, given off about 800 grammes. 



Oxygen, consumed about 700 u 



Water, given off about 500 " 



The amounts of O taken up and of CO 2 given off differ in dif- 

 ferent individuals and in the same individuals under varying 

 circumstances, among which the following may be enumerated: 



1. Increase in the rapidity or the depth of respiratory move- 

 ments, accompanied by an increase in the tidal stream, pro- 

 30 



